Methods for scanning an examination object using a CT scanner are well known. Therein, use is made of e.g. circular scans, sequential circular scans with feed, or helical scans. In these scans, at least one X-ray source and at least one opposing detector are used to collect measurement data in the form of absorption data from the examination object over an angular interval, and this data is operated on by way of appropriate reconstruction methods to form images, e.g. slice images.
These days, a so-called filtered back projection (FBP) method is used as a standard method for reconstructing the images. After the data acquisition, a so-called “rebinning” step is performed, in which the measurement data generated by the beam expanding in a fan-like fashion from the source is reordered such that it is available in a form as if the detector was hit by beams running parallel toward the detector. The measurement data is then transformed into the frequency space. Filtering takes place in the frequency space and the filtered data is subsequently subjected to an inverse transform. Using the measurement data resorted and filtered in this fashion, a back projection onto the individual voxels within the region of interest is then carried out.
A disadvantage in scanning an examination object subjected to or undergoing cyclical motion, for example a heart or a lung or an examination object at least partly subjected to cyclical motion, consists of the fact that motion unsharpness can be generated in the images in these calculation methods. This is because there can be, as a function of the recorded intensity of the motion, a spatial offset of the examination object or part of the examination object in the measurement data, which is recorded during the time of a scanning procedure and required for reconstructing the image, and so not all of the measurement data reflects spatially identical situations of the examination object. This motion unsharpness problem is particularly amplified when carrying out cardiac CT examinations of a patient, in which there is strong motion unsharpness in the cardiac region due to the cardiac motion.
In order to avoid such motion unsharpness, the measurement data is therefore recorded whilst evaluating a motion signal; in the case of the heart this is whilst evaluating an EKG signal transferred from the patient. Two reconstruction methods exist, which in principle differ in terms of their approach.
In the case of retrospective reconstruction, measurement data is recorded over the entire cycle duration of the cardiac motion and stored together with the EKG signal. An image is reconstructed retrospectively, following data acquisition, wherein measurement data of rest phases of the heart is selected either by evaluating the EKG signal or by evaluating the motion information contained in the data records using a well-known motion-mapping method. An advantage consists of the fact that the rest phases of the heart can be determined individually for each cycle, and so the reconstructed image has few motion artifacts. However, a requirement for the retrospective reconstruction of an image is that the patient is irradiated with the full X-ray dose throughout the entire scan, and so a significantly higher X-ray dose than necessary is applied. That is to say measurement data that is not required for the actual reconstruction is also recorded during the scan.
In the case of prospective reconstruction, a recording time for a temporally subsequent phase of the cardiac motion to be scanned is estimated on the basis of a registered R-wave and an established RR-interval duration. In the process, the R-wave of the EKG signal is used as a trigger to start the scanning after a delay predefined by the recording time. Thus, the recording time is the time interval between the registered R-wave and the start of measurement data acquisition within the subsequent motion cycle. It can selectively be specified in relation to the established RR-interval duration, e.g. in percent, or else in time units. The recording time in general is fixed by an operator using their experience and as a function of the observed characteristic in the EKG profile. A particular disadvantage of this is that the result of the image reconstruction depends strongly on the experience of the respective operator. Thus, what can occur is that the predefined recording time does not coincide with the phase or the rest phase of the cardiac motion in an optimum fashion, leading to a reduction in the obtainable image quality. However, an advantage of prospective reconstruction of tomographic images can be seen in the fact that the tube current is modulated such that the X-ray dose is only applied during a short time interval.
A further aspect increasing the difficulty of scanning is the fact that the cyclically moved examination objects in the patient, namely the heart or the lung, have attenuation properties similar to their surroundings, and so they are imaged with only little contrast in the image if no further measures are taken. It is for this reason that a contrast agent is injected into the patient before the examination is started. Compared to the surrounding tissue, the contrast agent has a different attenuation property, and so a significant contrast is generated in the image between the examination object and the surroundings.
The diffusion of the contrast agent in the body of the patient is a highly dynamic process. The concentration of the contrast agent in the examination object first of all increases sharply after a certain time delay, reaches a maximum, and subsequently falls off again. So that data is recorded at the right time, namely at the time of maximum concentration of the contrast agent in the examination object, a preliminary examination with a small amount of the contrast agent (test bolus) is first of all carried out at the beginning of each examination and the goal thereof is to establish the time profile of the contrast-agent concentration or the contrast-agent curve.
In the process, measurement data is recorded at a sequence of recording times, wherein a first image is reconstructed from the measurement data at each recording time and the images are used to establish a time profile of a concentration of the contrast agent.
In cardiac CT examinations, measurement data is recorded for example in the upper cardiac region of the ascending aorta at a fixedly predefined time interval, for example in a one-second interval. A slice image is reconstructed from the measurement data at each recording time. The time profile of the contrast agent concentration is subsequently established from the profile of the attenuation values from a ROI positioned in the slice images in the region of the aorta. The number of required scans depends individually on the patient and varies between 3 and 15.